Through investigations on the growth of the (100) face of potassium dihydrogen phosphate (KDP) crystal by ex situ atomic force microscopy (AFM) with different pH values at different supersaturations at 40°C,it was found that the growth of crystal was controlled by step flow at lower supersaturations,and the morphologies of steps were different under different growth conditions.In addition,at the higher supersaturations,2D nucleation mechanism controlled the growth.When the supersaturations were lower,the dislocation mechanism controlled the growth of crystal,and when σ≥0.05,2D nucleation mechanism played a dominant role in the growth of the (100) face for pH=4.2 and pH=2.5.However,for pH=5.0,the dislocation mechanism also dominated the growth of crystal when the supersaturations were lower,but when σ≥0.03,the crystal growth was controlled by 2D nucleation mechanism.Through investigations on the step flow of the (100) face of KDP crystal by in situ AFM with different pH values at lower supersaturations at 25°C,the velocities of normal growth of the (100) face were estimated at different growth conditions by in situ AFM.It was found that when the pH value was 5.0,the normal growth rate was the fastest at the same supersaturation compared to the other pH values and screw dislocation mechanism controlled the crystal growth.In addition,we found that with the reduction of the supersaturation of the solution,the density of steps also decreased,the width of steps became larger.Finally,a phenomenon which was the obvious anisotropic growth of steps in the step flow was observed by in situ AFM at σ=0.025 at pH=5.0. Through investigations on the growth of the (100) face of potassium dihydrogen phosphate (KDP) crystal by ex situ atomic force microscopy (AFM) with different pH values ​​at different supersaturations at 40 ° C, it was found that the growth of crystal was controlled by step flow at lower supersaturations, and the morphologies of steps were different under different growth conditions. In addition, at the higher supersaturations, 2D nucleation mechanism controlled the growth. When the supersaturations were lower, the dislocation mechanism controlled the growth of crystal, and when σ≥0.05, 2D nucleation mechanism played a dominant role in the growth of the (100) face for pH = 4.2 and pH = 2.5.However, for pH = 5.0, the dislocation mechanism also dominated the growth of crystal when the supersaturations were lower, but when σ ≥ 0.03, the crystal growth was controlled by 2D nucleation mechanism. Through investigations on the step flow of the (100) face of KDP crystal by in situ AFM with different pH values ​​at lower supersaturations at 25 ° C, the velocities of normal growth of the (100) face were estimated at different growth conditions by in situ AFM. It was found that when the pH value was 5.0, the normal growth rate was the fastest at the same supersaturation compared to the other pH values ​​and screw dislocation mechanism controlled the crystal growth. Moreover, we found that with the reduction of the supersaturation of the solution, the density of steps also decreased, the width of steps became larger. which was the obvious anisotropic growth of steps in the step flow was observed by in situ AFM at σ = 0.025 at pH = 5.0.